Giant off-the-road tires, particularly those employed by earth moving and mining ore vehicles, are subjected to very heavy loads that make them susceptible to premature failure unless proper inflation pressures are maintained. For example, FIG. 1 shows a mining ore truck 10, which can weigh up to 550 tons when loaded and carries its heavy loads on only two axles. A front axle 12 has two single tires 14 mounted thereon and a rear axle 16 has two dual tires 18 (only one "dual" shown) mounted thereon, resulting in loads of about 50 tons per tire. A typical tire is inflated through a conventional valve stem 20 (only two shown) to an operating pressure ranging from about 85 to 185 pounds per square inch of gauge pressure ("psig") and, when operating, may have an operating temperature ranging from about 100 to 255 degrees Fahrenheit (".degree. F."). If the tire pressure is too high, a worst case failure mode (explosion) may occur. However, if the tire pressure is too low, the excess heat generated may cause separation of some of the 18 to 22 tire layers after only 300 hours of operation, whereas such tires normally have at least a 1,000 hour operating life. Giant off-the-road tires cost about $25,000 to $30,000 each, and vehicle downtime costs at least about $500 per hour. Clearly, maintaining proper tire operating pressure is an economic imperative.
Unfortunately, conventional tire pressure measurement methods require allowing the tires to cool for about eight hours before an accurate tire pressure measurement and inflation pressure adjustment can be made. The resulting costly downtime often leads to infrequent tire pressure measurements. To make matters worse, air compressors often add water vapor and compression heating of the inflation air, causing errors between the measured and operating air pressures in the tires. Moreover, off-the-road vehicle operators often add fluids to tires to inhibit rim rust and scale that otherwise makes tire removal difficult. Also, new tires are stored outdoors where they can collect rainwater, some of which inevitably remains in the tire when it is installed on a vehicle. Unfortunately, such fluids have vapor pressures that contribute to pressure measurement errors. Tires operating under these conditions are referred to as "wet" tires. Finally, many off-the-road vehicles operate in cold climates but are maintained in heated facilities, further complicating the tire inflation pressure problem. Unfortunately, conventional gas law equations do not provide solutions to these problems.
There are previously known apparatus and methods for solving some of the above-described problems. In particular, U.S. Pat. No. 5,452,608 for PRESSURE AND TEMPERATURE MONITORING VEHICLE TIRE PROBE WITH RIM ANCHOR MOUNTING describes a tire rim mounted sensor probe and conductor terminal apparatus for sensing the air pressure and temperature inside a tire. When the vehicle stops, a conventional electronic measuring device is electrically connected to the conductor terminal to convert the sensor probe data into pressure and temperature measurements. Unfortunately, the sensor probe is separately mounted through a hole in the rim, a disadvantage that weakens the rim and requires a relatively expensive field retrofit to every rim. If a new rim is required, it similarly has to be retrofitted, adding to the downtime of vehicle. Of course, a tire cool down period may be required to obtain usable readings.
U.S. Pat. No. 5,335,540 for TIRE MONITORING APPARATUS AND METHOD describes a tire pressure and temperature sensing apparatus that employs radio telemetry to continuously monitor tire pressure and temperature while the vehicle is operating. However, as in the above-described patent, the pressure and temperature sensing probe is separately mounted to the rim, which has many of the above-described disadvantages. Moreover, every vehicle carries a telemeter receiver by which the vehicle driver monitors the tire operating pressures and temperatures.
There are many other patents describing tire pressure and temperature monitoring apparatuses and methods. Some describe sensors embedded in the tires, others describe wheel hub mounted slip rings for conducting sensor data to a vehicle data processor, and still others describe complex systems for inflating and deflating tires while the vehicle is operating. However, there is no known prior method or apparatus that accounts for pressure measurement errors caused by air compressors, water vapor pressure, and temperature changes.
What is needed, therefore, is an accurate tire pressure measuring system that does not require a cool down time, does not require expensive retrofitting or weakening of tire rims, accounts for sources of measurement errors, is readily transferred among tires and vehicles, and is usable on a wide variety of tire and wheel combinations.